1. Field of the Invention
The invention relates generally to optical scanners, and in particular to scanners having dual or multiple working ranges.
Most optical scanners such as bar code scanners are adapted for use at a particular distance, or a range of distances, from an indicia to be scanned. If the user holds the scanner too close to the indicia, or too far away, the indicia and/or the flying spot beam will not be in focus, and decoding will not be possible.
Such scanners may not be particularly convenient in environments where a series of indicia to be read are presented to the scanner at various distances, and where it is difficult or impossible for the user to alter the distance between the scanner and the indicia. To deal with such situations, attempts have been made to expand the acceptable working range of conventional scanners, to give the user as much leeway as possible, and also to provide multi-distance scanners which can operate, for example, at a first working range or at a second working range according to the user's preference or requirements. One possibility is for the provision of a two-position switch on the scanner, with the scanner operating at a first working distance in a first position of the switch and at a second working distance in a second position. A disadvantage of such scanners is that they require additional moving parts to provide for operation at the two separate working ranges. Such systems are also not "automatic" in the sense that the user has manually to select the correct working range, according to the distance of the current indicia to be read; if the incorrect working range is chosen, a decode will not result.
One of the difficulties that bar code reader designers face when attempting to produce increased working ranges is that the greater the working range, and the greater the range of possible indicia that might be read, the lower tends to be the resultant signal to noise ratio in light that is reflected from the indicia. One approach for dealing with this involves the provision of non-conventional optics, in which the optics associated with either the laser or with the photodetector have two distinct focal points. An example of this is shown in U.S. Pat. No. 5,332,892, which is commonly assigned with the present application. In the device shown in that document, the two focal points are associated with corresponding circuitry to provide two separate channels of data derived from the scanned bar code. The two channels have differing resolutions. As the working angle and density vary, at least one of the resolutions is likely to be appropriate for sensing all or most of the bar coded data, regardless of the distance of the bar code with respect to the scanner and/or the size or density of the code. The scanning beams of bar code readers are typically derived from laser diodes. Such diodes are robust and relatively inexpensive, but they do suffer from the disadvantage that the beam emerging from a laser diode is astigmatic. The astigmatic laser diode can be characterized as having two apparent light sources spaced apart from each other along the optical path. One of the light sources lies in a horizontal plane, appears to be coming from inside the laser diode chip, and has a low angular divergence. The other apparent light source lies in a vertical plane, appears to be coming from a facet of the chip, and has a high angular divergence. The two apparent light sources, which are spaced apart from each other by typically about 20 micrometers, form two beam waists in different planes and in different directions, as measured relative to the planar junction of the chip.
The resultant relatively complex beam profile may need selective shaping before it can efficiently be used in an optical scanner. Some methods of providing such beam shaping are described in our co-pending U.S. patent application Ser. No. 08/268,982, filed Jun. 30, 1994, the teachings of which are incorporated herein by reference.
A simpler option is simply to provide separate long and short range visible laser diodes, as is suggested in our earlier patent U.S. Pat. No. 5,420,411.
A further problem associated with known arrangements is that of distinguishing the respective images received corresponding to objects in each working range.
The present invention, in various other embodiments, further relates to optical scanning stations having a conveyor for moving articles carrying indicia to be read past an optical scanner, and to optical scanners having means for manually or automatically selecting one of a plurality of desired scan patterns.
The invention further relates, in yet further embodiments, to scanners having an extended working range using two lasers, each focused to cover a different portion of the scan distance. Present systems turn on one laser for a full scan, and then the second laser for the next scan, alternating on a scan-by-scan basis. Since, in general, only one of the lasers is capable of reading a bar pattern at any given distance, this technique typically doubles the length of time that it takes to generate a successful decode over single-laser embodiments. There is a need to regain the aggressiveness of a one-laser scanner, while maintaining the range benefits of the two-laser system. One device making use of two lasers is disclosed in our application Ser. No. 08/405,585, filed Mar. 17, 1995, now abandoned, the disclosure of which is incorporated herein by reference.